1. Field of the Invention
The present invention relates to a method of converting a microcapsule-containing water-based dispersion into a microcapsule-containing oil-based ink or oil-based coating liquid without coagulating the individually dispersed primary microcapsule particles.
More precisely, the method comprises blending a microcapsule-containing aqueous dispersion with a solvent such as glycerine which has an affinity for both a varnish for printing use or a resin for coating use, and the surface of the microcapsule wall material, and with an oil-based coating liquid comprising the foregoing oil varnish or resin, and removing the water present in the resulting mixture by vacuum distillation to convert the microcapsules in a water-based dispersion into those in an oil-based system without coagulating the primary dispersed microcapsules in order to obtain a microcapsule-containing oil-based coating liquid.
Examples of the foregoing oil-based coating liquid comprising an oil varnish for ink use or a resin for coating use include methacrylic derivatives, acrylic derivatives, mixed glycerides of an unsaturated fatty acid, and the like.
The present invention further relates to a sheet, in particular to a pressure-sensitive printing sheet of paper (noncarbon-type duplicating printing paper or so-called "carbonless paper") with a coating layer comprising a microcapsule-containing oil-based ink on a selected area of the printing sheet of paper, synthetic paper or plastic film, and to a method of preparing the same.
2. Background Art
Methods for preparing microcapsules are now applied in many industrial areas for encapsulating materials, such as dye, perfume, liquid crystal, pigment, solvent, adhesives to make these core materials easier to handle and to maintain their performance for a long period of time. These core materials are usually encapsulated in a water-based system; therefore, microcapsules in such a water-based system are very difficult to convert directly into an oil varnish or a resin-based coating liquid.
If s microcapsule-containing water-based dispersion could be directly converted into an oil-based one, such a process or an oil-based dispersion thus obtained could be applied for various kinds of fields such as roll milling of plastics such as polyvinyl chloride. Particularly, in the printing field, especially in a field of preparing pressusre sensitive dupulicating sheets, s special desensitizing printing ink capable of preventing coloring has hitherto been coated an a selected area of the sheet for making a carbonless sheet of paper or self-coloring sheets; however, in this case, when a color former core microcapsule-containing oil-based printing ink is used, direct printing for a selected area on the printing sheet will be possible preferably, even on a sheet of thin paper, sandwiched paper, or synthetic paper, these papers having never been printed before, as well as fine paper by utilizing the conventional desensitizing printing unit untouched.
Conventional microcapsulation of dye has been carried out in a water-based system; therefore, the resulting dye core microcapsule-containing ink is of a water-based composition, and has been limited in its uses. When a carbonless sheet of paper is made by coating a selected area of the sheet by flexographic printing with a color-developing dye core microcapsule-containing water-based coating liquid, the coating layer shrinks upon drying and sometimes causes wrinkles around the printed area because of a necessity of drying; therefore, such a water-based coating layer is difficult to apply freely. Such problems associated with the use of a microcapsule-containing water-based material have usually been circumvented by coating the entire surface with the layer of a color former by means of an air knife, and thereafter, applying a desensitizing ink on the unnecessary areas in order to prevent coloring. This is obviously tedious. In addition, when the amount of the desensitizing ink is insufficient and the printing is uneven, coloring takes place due to the inefficient preventing effect, showing that a large excess of desensitizing ink is necessary. Owing to the higher tack and lower flow of the desensitizing ink comparing with other conventional ink, it becomes necessary for a specifically designed unit to increase the amount of the desensitizing ink to be printed or the printing speed needs to be lowered, indicating higher operating costs and lower productivity. Further, the amount of microcapsule-containing water-based ink to be coated is usually limited to the range wherein curling of the sheet does not take place.
On the other hand, when a plastic sheet is printed with such a microcapsule-containing water-based coating liquid, setting properties of the coating (i.e. fixing, curing, etc) becomes unsatisfactory, showing that plastic sheets can not be employed for such a purpose. A sheet of art paper, coat paper, gravure paper, synthetic paper, thin paper, or sandwiched paper likewise has been found to be actually difficult to use for making an upper printing or intermediate printing sheet of a carbonless sheet of paper.
In order to circumvent these problems, a process has been practiced, wherein dried microcapsule powder with good flowability is milled into an oil varnish or a resin for coating use after a microcapsule-containing water-based dispersion is subjected to a conventional spray drying to obtain the dried microcapsule powder, because a microcapsule-containing water-based dispersion cannot be converted directly into an oil-based varnish for printing ink use.
In this spray drying process, however, it becomes necessary to increase the amount of ink to disperse the powdered microcapsules to obtain ink of suitable flowability. As a result, it is difficult to achieve high concentrations of microcapsules, indicating a problem of insufficient coloring of the resulting print. Besides, the primary microcapsule particles are sometimes converted into secondary particles by coagulation upon spray drying in the presence of a dried emulsifying agent derived from and used in the original polymerization system for preparing the microcapsule wall. A dried solid film of the emulsifying agent covers around the surface of the secondary particles. Because of this wrapping, the particle size of the coagulated secondary particles increases by a factor of, for example, from several to several 10, unpreferably. These large secondary particles have to be subjected to crushing to provide particles with a suitable average particle size of less than 2 .mu.m, preferably less then 1 .mu.m in order to obtain suitable printability. Further, the microcapsule wall material becomes brittle by hardening upon spray drying and the walls of the secondary particles are broken down with ease upon the crushing. As a result, when a carbonless sheet of intermediate paper coated with a color developing dye on the back and a developer on the surface is made, the color developing dye of the core material freed from encapsulation penetrates into the fiber matrix of the paper while causing coloring of the surface which has been coated with the developer unpreferably.
In such a process, in addition to high costs due to the necessity of installing a spray dryer, a blender, crushing rolls, and the like, the productivity becomes too low because a substantial amount of microcapsules is lost upon spray drying, wherein a diluted microcapsule-containing water-based dispersion, for example, a 5% dispersion has to be reluctantly employed in order to bring the particles closer to primary particles.
So-called "Flushing Process" has been practiced in the ink manufacturing industry to convert a water-based colorant into an oil-based colorant, wherein the process comprises mixing a cake-like colorant with an oil varnish, elevating the temperature to remove the supernatant water by decantation, continuing heating and vacuum distillation of the residual water to convert the water-based colorant into an oil-based one. When a colorant-containing water-bused dispersion is directly employed, problems associated with solidification of the colorant, separation from the varnish, foaming upon evaporation, or bumping, and the like arise; therefore, the dehydrated cake-like colorant is usually employed after the colorant contained in the water-based dispersion is filterized and pressed.
Such a "Flushing Process" has been found to be not practical for application to the present invention, because he foregoing cake-like colorant particles consist mainly of coagulated secondary particles, and have to be further subjected to crushing by means of, for example, a crushing mill to provide it with suitable printability.